UDP: join multicast group on the interface specified with &localaddr=

When joining multicast groups, ffmpeg was using INADDR_ANY as interface address
which leads to enabling the multicast group on the interface with "default gateway".
Often multicast traffic is received over dedicated interface, which scenario ffmpeg was
unable to handle. With this patch, ffmpeg will enable multicast group to the interfaces
configured with address specified in &localaddr= parameter of udp:// URL. To avoid
loacal_addr resolve at udp_close(...) the UDPContext structure was extended with
struct sockaddr_storage local_addr_storage member, which is populated in udp_open(..)
and passed to udp_join_multicast_group()  and udp_leave_multicast_group().
Signed-off-by: Stoian Ivanov <s.ivanov@teracomm.bg>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/udp.c

@@ -84,6 +84,7 @@ typedef struct {
     char *local_addr;
     int packet_size;
     int timeout;
+    struct sockaddr_storage local_addr_storage;
 } UDPContext;
 
 #define OFFSET(x) offsetof(UDPContext, x)
@@ -140,13 +141,16 @@ static int udp_set_multicast_ttl(int sockfd, int mcastTTL,
     return 0;
 }
 
-static int udp_join_multicast_group(int sockfd, struct sockaddr *addr)
+static int udp_join_multicast_group(int sockfd, struct sockaddr *addr,struct sockaddr *local_addr)
 {
 #ifdef IP_ADD_MEMBERSHIP
     if (addr->sa_family == AF_INET) {
         struct ip_mreq mreq;
 
         mreq.imr_multiaddr.s_addr = ((struct sockaddr_in *)addr)->sin_addr.s_addr;
+        if (local_addr)
+            mreq.imr_interface= ((struct sockaddr_in *)local_addr)->sin_addr;
+        else
             mreq.imr_interface.s_addr= INADDR_ANY;
         if (setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (const void *)&mreq, sizeof(mreq)) < 0) {
             log_net_error(NULL, AV_LOG_ERROR, "setsockopt(IP_ADD_MEMBERSHIP)");
@@ -169,13 +173,16 @@ static int udp_join_multicast_group(int sockfd, struct sockaddr *addr)
     return 0;
 }
 
-static int udp_leave_multicast_group(int sockfd, struct sockaddr *addr)
+static int udp_leave_multicast_group(int sockfd, struct sockaddr *addr,struct sockaddr *local_addr)
 {
 #ifdef IP_DROP_MEMBERSHIP
     if (addr->sa_family == AF_INET) {
         struct ip_mreq mreq;
 
         mreq.imr_multiaddr.s_addr = ((struct sockaddr_in *)addr)->sin_addr.s_addr;
+        if (local_addr)
+            mreq.imr_interface= ((struct sockaddr_in *)local_addr)->sin_addr;
+        else
             mreq.imr_interface.s_addr= INADDR_ANY;
         if (setsockopt(sockfd, IPPROTO_IP, IP_DROP_MEMBERSHIP, (const void *)&mreq, sizeof(mreq)) < 0) {
             log_net_error(NULL, AV_LOG_ERROR, "setsockopt(IP_DROP_MEMBERSHIP)");
@@ -624,6 +631,8 @@ static int udp_open(URLContext *h, const char *uri, int flags)
     if (udp_fd < 0)
         goto fail;
 
+    s->local_addr_storage=my_addr; //store for future multicast join
+
     /* Follow the requested reuse option, unless it's multicast in which
      * case enable reuse unless explicitly disabled.
      */
@@ -668,7 +677,7 @@ static int udp_open(URLContext *h, const char *uri, int flags)
                 if (udp_set_multicast_sources(udp_fd, (struct sockaddr *)&s->dest_addr, s->dest_addr_len, include_sources, num_include_sources, 1) < 0)
                     goto fail;
             } else {
-                if (udp_join_multicast_group(udp_fd, (struct sockaddr *)&s->dest_addr) < 0)
+                if (udp_join_multicast_group(udp_fd, (struct sockaddr *)&s->dest_addr,(struct sockaddr *)&s->local_addr_storage) < 0)
                     goto fail;
             }
             if (num_exclude_sources) {
@@ -838,7 +847,7 @@ static int udp_close(URLContext *h)
     int ret;
 
     if (s->is_multicast && (h->flags & AVIO_FLAG_READ))
-        udp_leave_multicast_group(s->udp_fd, (struct sockaddr *)&s->dest_addr);
+        udp_leave_multicast_group(s->udp_fd, (struct sockaddr *)&s->dest_addr,(struct sockaddr *)&s->local_addr_storage);
     closesocket(s->udp_fd);
 #if HAVE_PTHREAD_CANCEL
     if (s->thread_started) {